Due to present environmental concerns over the use of ozone-depleting blowing agents, it is desirable to make alkenyl aromatic polymer foams with blowing agents having reduced or zero ozone-depletion potential. Such blowing agents include inorganic blowing agents such as nitrogen, sulfur hexafluoride (SF.sub.6), and argon; organic blowing agents such as carbon dioxide and hydrofluorocarbons such as 1,1,1,2-tetrafluoroethane (HFC-134a), 1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1,3,3-pentafluoropropane, difluoromethane (HFC-32), 1,1-difluoroethane (HFC-152a), pentafluoroethane (HFC-125), fluoroethane (HFC-161) and 1,1,1-trifluoroethane (HFC-143a) and hydrocarbons such as methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, cyclopentane and neopentane; and chemical blowing agents which include azodicarbonamide, azodiisobutyro-nitrile, benzenesulfonylhydrazide, 4,4-oxybenzene sulfonyl-semicarbazide, p-toluene sulfonyl semi-carbazide, barium azodicarboxylate, N,N'-dimethyl-N,N'-dinitrosoterephthalamide, trihydrazino triazine and mixtures of citric acid and sodium bicarbonate such as the various products sold under the name Hydrocerol.TM. (a product and trademark of Boehringer Ingelheim). All of these blowing agents may be used as single components or any mixture in combination thereof, or in mixtures with other co-blowing agents.
A problem with using the above non ozone-depleting blowing agents is their tendency to form foams of relatively small cell size and cross-section. Such blowing agents typically result in foams having small cell sizes due to their relatively high nucleation potential. Small cell size is especially a problem when particular infrared attenuating agents are employed such as carbon black, graphite, and titanium dioxide.
It would be desirable to be able to employ the non-ozone depleting blowing agents in making alkenyl aromatic polymer foams with or without infrared attenuating agents yet be able to enlarge the cell size of the foam. Enlarging the cell size of the foams would enable greater thicknesses and larger cross-sectional areas to be obtained, as well as afford a reduction in foam density in some cases. Lower foam densities would be desirable for both extruded and expanded alkenyl aromatic polymer foams. Greater foam thicknesses and cross-sections would enable a broader range of products to be manufactured, and reducing density would allow foams to be manufactured more economically. It is also desirable for the foams to exhibit acceptable physical properties.
Prior art attempts to make a foam having enlarged cell size include the integration of a wax in a foam forming gel prior to extrusion of the gel through a die to form a foam. Such use of a wax is seen in U.S. Pat. No. 4,229,396, which is incorporated herein by reference. The use of a wax may however, present processing problems and cause thermal stability variations or diminution in physical properties in product foams. The wax may also cause inconsistency in extrusion temperatures. Additional prior art attempts to make a foam having enlarged cell size include the incorporation of a non-waxy compound in a foam forming gel prior to extrusion of the gel through a die to form a foam. Such use of a non-waxy compound is seen in U.S. Pat. No. 5,489,407, which is incorporated herein by reference. Large cell size alkenyl aromatic polymer foams have been prepared using glycerol monoesters of C.sub.8 -C.sub.24 fatty acids as cell size enlarging agents as disclosed in U.S. Pat. No. 5,776,389, the entire contents of which are incorporated herein by reference. However the concentration of such agents in a foam that can be used is limited, as high levels depress the glass transition temperature of the polymer and can result in degradation of physical properties such as creep under load (at 80.degree. C.).
Thus it would be desirable to identify cell size enlarging compounds which can be used in conjunction with non-ozone depleting blowing agents and do not have an adverse effect on the physical or mechanical properties of the foam.